This invention relates to vehicle wheel balancers and, in particular, to a wheel balancer having a quick calibration check to determine if the balancer is properly calibrated.
An imbalanced vehicle wheel mounted and rotating on a wheel balancer generates sinusoidal forces which are detected by piezoelectric crystals, strain gauges, or other suitable transducers of the wheel balancer. Dynamic wheel balancers detect these sinusoidal forces to determine the magnitude of the imbalance. A computer receives the output of the crystals or gauges, and in response to input of the wheel parameters, determines the weight magnitudes and weight locations to correct the imbalance for two correction planes of the wheel. The correction planes are typically located at the inner and outer wheel rim lips. The wheel parameters include the distance from a reference point to a first plane of the wheel, the width of the wheel (to determine the location of a second plane of the wheel), and the radius of the wheel.
To properly determine the imbalance of a wheel, the balancer must have first been calibrated. The balancer performance can be affected by "drift" of the transducer, i.e., the transducer produces a different response (output per unit of force) than it created during calibration. Other factors, such as age, temperature, and the mechanical environment, can also effect the performance of the balancer. If a balancer is not properly calibrated, it will not properly determine the imbalance in a wheel. This leads to the output of erroneous data about the weight and weight location needed to correct the imbalance condition. Even after a balancer has been calibrated, the operating conditions can change, requiring the balancer to be recalibrated.
When a wheel balance is performed, and a weight is placed on the wheel to correct for the wheel's imbalance, a second spin is usually performed to determine that the wheel is balanced. When a balancer is out of calibration, the transducer outputs a signal that is off by some factor. When this second spin is performed with the balancer is out of calibration, it will signal that the wheel is still unbalanced. This requires that the operator bracket or "chase" the weights, i.e. perform multiple balancing operations on the same wheel to properly balance the wheel. This is the only indication from a balancer which can inform the operator that the balancer is out of calibration and needs calibrating. As can be appreciated, this accidental determination of the balancer's calibration status is undesirable. It can lead to significant delays in the balancing of a wheel while the weights are chased, the wheel is dismounted to calibrate the balancer, and the wheel is remounted to be balanced. To avoid this, some operators calibrate their balancer on a regular basis, for example weekly. This too, however, takes time. What is desirable is some method to quickly check to determine if the balancer is within calibration.
U.S. Pat. No. 4,441,355, to Rothamel, discloses an automatic, self-calibrating balancer. That is, it corrects for any drift in the transducers automatically. This balancer avoids the need to perform calibration checks. The Rothamel balancer has a secondary shaft to which a known imbalance is imparted. This secondary shaft operates at a different frequency than the shaft on which the wheel/tire assembly is mounted. The balancer measures the imbalance forces of this secondary shaft and compares it with predetermined nominal data. The output, supplied by a comparator, is used to correct the signal from the primary shaft to correct for any change in the drift of the transducers. However, the addition of the second shaft, with its associated weights and gears, adds cost to the balancer unit. Further, the operation of the secondary shaft creates undesirable audible noise.
U.S. Pat. No. 4,250,555, to Mitchell et al., also discloses a self calibrating balancer. Mitchell et al use four strain gauges as the force pick-ups to detect the imbalance of a rotating wheel. The output from the strain gauges is passed through a series of pre-amps and analog filters. Mitchel et al. calibrate the pre-amps and filters for every spin by forcing a square wave through the analog electronics. The output of the electronics is compared to an expected result to determine a correction factor which is applied to the output from the filters and the pre-amps. Mitchel et al. check the analog components for drift. However, they do not check the strain gauges themselves for drift or other problems which may affect their output. If the output from the strain gauges of the Mitchel et al. device were incorrect for any reason, the error would not be detected by the device calibration system disclosed. This is obviously undesirable in a system in which the force pick-ups can be affected by temperature and mechanical changes.